Every parents' hope when a child has a debilitating disease a potentially life saving drug. But what happens when you have two children with the same disease and only one gets the medicine. Here's ABC's doctor Richard best. Two -- in the rear form of cystic fibrosis one gets better the other doesn't. -- part of a study for new drugs it has the potential to dramatically change their lives. But as in most -- only half the patients get the real. We didn't know they are both sugar hill are they are both -- the drug -- one and one of the parents of exactly the same but within weeks the girls weren't acting this. Isley isn't -- me as much as -- was at that time. And now many -- said that getting a lot more energy in is going a lot better she had. -- tweaking her chest was clear. And that the thing that Lewis Holloway was at her lung function test skyrocketed because of cystic fibrosis -- -- Laura have damaged lungs. Persisted finally caught in -- don't grow as well. Because of the genetic defects their bodies make an abnormal protein that prevents salt and water for moving through cell walls. Creating a sticky mucus that -- their lungs. This new drug called Politico repairs -- broken protein so what does this mean for the girls it means vacant factory. They are -- now on the drug. Very good -- jump on trampoline without tiring part yet -- dance without brakes police soccer and enjoyed just being normal preteens. For their mom it means she can dream. What's the future look like that now. My wish is my daughters -- -- him as any parent I want them to go to college I want them to get married I want can have healthy children I think it completely changes their future. Doctor Richard -- ABC news North Andover Massachusetts.

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